The brouhaha over a moderate New Jersey mayor’s comments has aggravated tensions that have been growing within the party’s coalition.

Newark Mayor Cory Booker’s off-message criticism of the Obama campaign’s attacks on Mitt Romney’s background at Bain Capital gave the campaign an untimely, unwanted headache this week. But more significantly, it exposed a tension that’s developing between the Democratic Party’s centrist wing and its more-outspoken liberal base—one that threatens to fester more openly if President Obama fails to win a second term.

Conversations with liberal activists and labor officials reveal an unmistakable hostility toward the pro-business, free-trade, free-market philosophy that was in vogue during the second half of the Clinton administration. Former White House Chief of Staff William Daley, who tried to steer the Obama administration in a more centrist direction, is the subject of particular derision. Discussion of entitlement reforms, at the heart of the GOP governing agenda, is a nonstarter. The fiscally conservative Blue Dog Democrats are now nearly extinct on Capitol Hill.

Moderate Democratic groups and officials, meanwhile, privately fret about the party’s leftward drift and the Obama campaign’s embrace of an aggressively populist message. They’re disappointed that the administration didn’t take the lead advancing the Simpson-Bowles deficit-reduction proposal, they wish the administration’s focus was on growth over fairness, and they are frustrated with the persistent congressional gridlock. Third Way, the centrist Democratic think tank, has been generating analyses underscoring the need for Democrats to appeal to middle-of-the-road voters, to no avail.

“There are not a lot of moderates left in the Democratic Party, and Cory is one of the few of them left,” said former Democratic Rep. Artur Davis, an early Obama ally who has become increasingly estranged from the party. “I would like to think Cory speaks for a lot of voters in the Democratic Party, but sadly he doesn’t speak for a lot of Democratic operatives within the party. This isn’t Bill Clinton’s Democratic Party anymore.”

The Booker brouhaha exposed an important dynamic. Booker is a leading figure in the centrist faction, and he didn’t hide that fact when he called out the Obama campaign attacks against Romney’s private-equity work. He first made a name for himself by challenging the party’s establishment in the 2002 Newark mayoral race, taking on the cronyism that was endemic in city government. After being elected in 2006 on his second try, he’s worked closely with the business community to revitalize Newark. He’s developed an unlikely odd-couple relationship with Republican Gov. Chris Christie, and the two have rallied behind education reform, taking on the teachers’ unions in the process.

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The Booker governing model, premised on bipartisanship and taking on ideological party factions, runs contrary in many ways to Obama’s record. It’s why Booker’s call for Obama to elevate the rhetoric drove Chicago batty: It was a stinging reminder that the candidate’s promise of a post-partisan approach in 2008 had given way to the reality of governing in a polarized Washington and the necessity of running a highly negative campaign against Romney.

It’s easy to ascribe political motivations to Booker’s apostasy, given that he has been mentioned as a prospective candidate for governor in 2013 against Christie or, more likely, for the Senate in 2014 to replace Democrat Frank Lautenberg (who may retire). And the investor set has certainly embraced his mayoralty, helping to fund his campaigns against business-unfriendly candidates. He will need that support in the future.

But Booker’s comments reflect an uncomfortable ideological divide within the elite elements of the Democratic coalition. As I wrote last month, wealthy voters made up a pivotal part of Obama’s winning coalition in 2008—not to mention contributing a good chunk of his campaign cash that cycle. But as Obama’s rhetoric has turned populist to capitalize on Romney’s biographical vulnerabilities, he has alienated some of those voters. Booker isn’t the only high-profile Democrat with Wall Street connections raising questions about the Obama campaign’s populist appeals. It follows similar criticisms from Obama auto adviser Steven Rattner, former Rep. Harold Ford Jr., and JPMorgan Chase’s Jamie Dimon, a past Obama donor and supporter. Meanwhile, Wall Street money has dried up, playing a role in the president’s less-than-imposing fundraising totals for the year.

Over the past generation, losing Democratic presidential campaigns have sparked soul-searching within the party. After Walter Mondale’s landslide defeat in 1984, centrist Democrats formed institutions, like the Democratic Leadership Council, to counter the influence of the old, liberal guard. The DLC later provided the governing fuel for Clinton’s presidency. A decade later, when John Kerry lost to George W. Bush, it sparked the rise of the liberal netroots, which played a key role in the Democratic takeover of Congress two years later. The party’s activist base currently is firmly in charge, but that could change quickly if things turn south.

With Obama at the helm of the party and broadly popular among Democratic voters, there’s little incentive for Democratic critics to openly take on a sitting president. The harsh reaction to Booker’s mild apostasy on Meet the Press, and his subsequent walk-backs, was proof positive of that. But if Obama loses to Romney after embracing a message advocated by the party’s liberal wing, don’t be surprised if the knives come out in force after the election.